Try this link, for $74.00 you get the refurbished meter and socket to plug it into. This is for the 120V 100 amp meter (will work with your 30 amp power). I believe you would use:
120 volt AC, 100 amp
M132 9 lbs $55
100 amp Socket, round housing, 1" threaded hub on top and bottom, 4 terminals, ground. 7" x 3" M138 4 lbs $19
Meters
Wire into the meter and put your plug receptacle on the other side. Then when you plug in, it will track your Kilowatt Hour usage. Other portable Kilowatt meters would cost you significantly more.
I have an RV lot that I own that used to be part of an RV Park that had no individual meters. When they went Condo, they put this type of unit on each power pole at the park. It is considered a sub-meter. The park still has all of the power coming through their commercial meters and pay for all of the power. They read the sub-meters each month to charge each owner for the actual electricity each uses.